1. Field of the Invention
The present invention relates to a vertically alignment liquid crystal display (LCD).
2. Description of the Related Art
Flat panel displays such as LCD, organic electroluminescence (EL), or plasma displays have been enthusiastically developed and commercialized in recent years. Particularly, LCDs has become a main display for office automation (OA) devices and audio visual (AV) devices because LCDs have attractive features such as thin and low power consumption. Especially, active matrix LCDs having thin film transistors (TFTS) as switching elements for controlling a timing to rewrite pixel data into each pixel enable a wide screen and animation display with a high resolution, and have become widely used in various television sets, personal computers, mobile computers, and monitors for digital still and video cameras.
A TFT is a kind of field effect transistor (FET) made of metal and semiconductor layers formed in a predetermined pattern on an insulated substrate. In an active matrix LCD, each TFT is connected to a corresponding capacitor for driving the liquid crystal disposed between a pair of substrates; the capacitor is constructed between the substrates.
FIG. 1 is an enlarged plan view of a display pixel portion of an LCD, and FIG. 2 is a cross section of the LCD along B--B line shown in FIG. 1. On the substrate 50 a gate electrode 51 is formed that is made of Cr, Ti, Ta, or another suitable metal, over which a gate insulating film 52 is formed. On the gate insulating film 52 an amorphous silicon, i.e., a-Si film 53 is formed in an island shape so as to cross over the gate electrode 51. On the a-Si film 53 an N.sup.+ a-Si film 53N is formed, each end of which is doped with impurities so as to make an ohmic layer. Above the channel region of the a-Si film 53, an etch stopper 54 is remained. On the N.sup.+ a-Si film 53N a drain electrode 56 and a source electrode 57 are formed, over which an interlayer insulating film 58 is formed. On the interlayer insulating film 58 a pixel electrode 59 that is made of indium tin oxide (ITO) or Al is formed, which is connected to the source electrode 57 via a contact hole formed in the interlayer insulating film 58. On the pixel electrode 59 an alignment film 71 made of polyimide or the like is formed, and is processed by rubbing treatment as shown in FIG. 3. In this way, the TFT substrate is manufactured.
On another substrate 60 facing the TFT substrate 50, red (R), green (G), and blue (B) color filters 61 are formed, each of which is made of a film resist and is disposed at a position corresponding to each pixel electrode 59. In addition, a black matrix 61BM which is made of a light shielding film resist is formed at a position corresponding to a gap between the pixel electrodes 59 and at a position corresponding to the TFT. On the layers of these color filters 61 and the black matrix, a common electrode 62 made of ITO is formed. On the common electrode 62 an alignment film 72 is formed and is processed by rubbing treatment in the same way as on the substrate 50. In this way, the opposing substrate is manufactured.
Between the TFT substrate 50 and the opposing substrate 60, a liquid crystal layer 80 is disposed. The orientation, i.e., the alignment of the liquid crystal molecules 81 is controlled in accordance with an intensity of an electric field formed by a voltage applied between the pixel electrodes 59 and the common electrode 62. Outsides of the substrates 50 and 60 polarizing films (not shown) with perpendicular polarizing axes are provided. Linear polarized light passing through these polarizing films is modulated when passing through the liquid crystal layer 80 that is controlled in different alignments per each display pixel, and is thereby controlled in a desired transmittance.
In the above-mentioned example, the liquid crystal has a negative dielectric constant anisotropy. The alignment films 71 and 72 are vertical alignment films that control the initial direction of the liquid crystal in the direction perpendicular to the substrate. In this case, when a voltage is not applied, the linear polarized light that passed through one of the polarizing films is blocked by the other polarizing films after passing through the liquid crystal layer 80 so that the display is recognized as black. When the voltage is applied, the linear polarized light that passed through one of the polarizing films is double refracted by the liquid crystal layer 80 to become an elliptically polarized light, which passes the other polarizing films so that the display is recognized as nearly white. This type is called a normally black (NB) mode. Particularly, the vertical alignment films 71 and 72 are processed by the rubbing treatment, so that the initial orientations of the liquid crystal molecules 81 are aligned in the direction with a slight pretilt from the normal direction. This pretilt angle .theta. is normally set to more than one degree, but equal to or less than five degrees. The liquid crystal molecule 81 is electrically uniaxial. The angle between the axial direction and the direction of the electric field is determined by the electric field strength, while the azimuth with respect to the direction of the electric field is not controlled. The liquid crystal molecule 81 having the negative dielectric constant anisotropy tilts in a direction different from the electric field direction. However, by providing pretilt, an applied voltage can make the liquid crystal molecule 81 tilt toward the pretilt direction. Therefore, the tilt directions are aligned so that a variation of alignments of the liquid crystal in a plane can be suppressed and deterioration of the display quality can be prevented.
The black matrix 61BM is provided for preventing a drop of the contrast ratio due to undesired light that is emitted from the display by the birefringence of the liquid crystal with the pretilt and passes through the liquid crystal layer 80 in a region in which the voltage is not applied between the display pixels.
FIGS. 4A-4E show a method for manufacturing the opposing substrate. First, in the step shown in FIG. 4A, the R, G and B color filters 61R, 61G and 61B are formed on the substrate 60. In order to form the R color filter 61R, an R film resist is affixed, which is then exposed and developed in the shape corresponding to the R display pixels. The G color filter 61G and the B color filter 61B are formed in a similar manner. These color filters 61R, 61G, and 61B are formed in dimensions slightly smaller than those of the corresponding display pixels 59 shown in FIG. 2.
In the next step shown in FIG. 4B, a light shielding film resist 61BM' is affixed, and is followed by the step shown in FIG. 4C, in which the film resist is exposed and developed in the shape corresponding to the gap between the pixels so that the black matrix 61BM is formed among the color filters 61R, 61G and 61B. This black matrix 61BM is formed in a dimension larger than the gap between the pixel electrodes 59 shown in FIG. 2.
In the next step shown in FIG. 4D, the ITO film is formed so as to produce the common electrode 62. In addition, in the step shown in FIG. 4E, a polyimide film is formed by a printing method. Then, the polyimide film is dried by baking, and processed by rubbing treatment. The film is rubbed in the arrow direction with a cloth so as to make the alignment film 72 for giving the pretilt to the liquid crystal.
The liquid crystal having a negative dielectric constant anisotropy changes the alignment of its molecules upon the electric field in such a way that the alignment becomes perpendicular to the direction of the electric field. On this occasion, the liquid crystal generates an action opposing the generated electric field. Generally, however, such a change of the orientation from the vertical alignment of the liquid crystal is not stable compared with a liquid crystal having a positive dielectric constant anisotropy such as a twist nematic (TN) liquid crystal changes from the horizontal alignment. Especially, unevenness of the alignment film 71 and 72 at the interface with the liquid crystal layer 80 due to a step of the TFT or the color filter influences the alignment change of the liquid crystal molecules 81, resulting in a deteriorated display quality.
Furthermore, as shown in FIGS. 3 and 4E, the related art uses a rubbing treatment for the vertical alignment film 71 and 72 in order to give the pretilt .theta. to the initial orientation of the liquid crystal as shown in FIG. 2. Therefore, when the voltage is applied, all the liquid crystal molecules 81 tilt in the direction of the pretilt (rightward in FIG. 2). Accordingly, the tilt angle of the liquid crystal molecule 81 with respect to the optical path when viewing the LCD from upper right in FIG. 2 is different from that when viewing the LCD from upper left, resulting different transmittances. Thus, there is a problem that a brightness or a contrast ratio changes in accordance with a viewing direction. This is called viewing angle dependence.
Furthermore, since the black matrix 61BM formed on the opposing substrate 60 side should cover the gap region between the pixel electrodes completely, it is formed larger in consideration of position shift when the black matrix 61BM is affixed to the TFT substrate 50 side. For this reason, effective display area decreases and aperture ratio decreases.
In addition, rubbing treatment for making the vertical alignment film 71 of the TFT substrate side may cause an electrostatic breakdown of the TFT, which results in defective display or decline of yield in production of LCDs.